method of manufacturing resinous papers, resinous papers, and their use as resinous preform blanks

ABSTRACT

The present invention relates to resinous papers, their use as resinous preform blanks and to a method of manufacturing resinous papers. The resinous paper or preform blank is made by preparing an aqueous fiber suspension having a consistency between 0,5-8%, preparing an aqueous dispersion of a thermosetting resin, mixing the aqueous dispersion of the thermosetting resin into the aqueous fiber suspension, preparing an aqueous dispersion of an agent for fixing the thermosetting resin on the fibers of the fiber suspension, mixing the aqueous dispersion of the fixing agent into the aqueous fiber suspension, adjusting a pH value of the aqueous fiber suspension to below 5, transferring the aqueous fiber suspension to a paper machine, dewatering the fiber suspension for the formation of paper, and drying the paper to a volatile content of 5% to 10%.

The invention relates to resinous papers, their use as resinous preform blanks and to a method of manufacturing resinous papers.

Papers impregnated or coated with resins or synthetic resins are frequently used for coating materials, in particular for coating of derived wood materials. Subsequently, the impregnated or coated papers are usually pressed onto the material under the application of pressure and heat. Typical coating methods are high-pressure laminating methods (HPL) or continuous laminating methods (CPL).

Impregnated raw papers are generally used today, which are impregnated with a thermosetting synthetic resin, such as a phenolic resin. Impregnating is carried out by subsequent immersing or coating of the raw paper with a liquid, aqueous or solvent-containing synthetic resin solution. The impregnated or coated paper is dried to a residual volatile content of about 6% to 9%. The surface of the paper is thus no longer sticky. it can therefore be manipulated and stored. At the same time a certain reactivity of the synthetic resin remains, which enables pressing of the resin-coated paper onto the material. So-called core papers are manufactured having a phenolic resin content of 30 to 70 weight % of the solids content of the finished resinous paper. These core papers are then cut to format for the HPL process and stored. For the CPL process, the core papers are processed from a roll.

Core papers known in the art can only be produced within a certain limited spectrum. The weight of the raw paper cannot be varied at will, and a certain raw paper can only be coated with a limited amount of synthetic resin. The upper weight limit of impregnated core papers is about 350 g/m². If a material is to be provided with a particularly resistive coating, a plurality of core papers must be superimposed in such cases and pressed onto the material. This is disadvantageous because multi-layered products can only be pressed with difficulty.

It is therefore an object of the present invention to provide a resinous paper which is more variable than prior art products with respect to the fiber content and the synthetic resin content of the paper.

This object is solved by improved resinous papers, by the use of the resinous papers as resinous preform blanks and by a method for manufacturing resinous paper. The method described hereinafter is a preferred method for manufacturing the inventive resinous papers or form blanks; however, the manufacturing of these products is not limited to the depicted method.

The inventive method comprises the established steps of:

-   -   preparing an aqueous fiber suspension having a consistency         between 0.5-8%,     -   preparing an aqueous dispersion of a thermosetting resin     -   mixing the aqueous dispersion of the thermosetting resin into         the aqueous fiber suspension,     -   preparing an aqueous dispersion of an agent for fixing the         thermosetting resin on the fibers of the fiber suspension,     -   mixing the aqueous dispersion of the fixing agent into the         aqueous fiber suspension,     -   adjusting a pH value of the aqueous fiber suspension to below 5,     -   transferring the aqueous fiber suspension to a paper machine,     -   dewatering the fiber suspension for the formation of paper,     -   drying the paper to a volatile content of 5% to 10%.

First, an aqueous fiber suspension is prepared. Natural and/or synthetic fibers or mixtures of these can be used. In practice, usually cellulose fibers or recycled paper fibers or a mixture of these are used. Other natural fibers like hemp or cotton fibers can be used, too. Synthetic fibers, such as viscose or aramide fibers, are also used; however, this is rare due to cost considerations. It is mentioned explicitly that wet laid products e.g. preform blanks that are completely or partially made of synthetic fibers, are “papers” in the sense of this method.

The fiber concentration of the suspension is mostly between 0.5 and 8%, preferably between 1% and 5%. An aqueous dispersion of a thermosetting resin, e. g. a phenolic resin, a melamine resin or a formaldehyde resin or a mixture of thermosetting resins is mixed into the fiber suspension. In contrast to the state of the art, the synthetic resin is not subsequently applied to the finished paper. It is intermixed intensively and homogeneously with the fibers prior to sheet formation. This significant feature of the inventive method, to mix the synthetic resin with the fibers prior to sheet formation, can be realized technically particularly easily. No complex modifications to the equipment are necessary. By the inventive use of aqueous resin dispersions, properties of the core paper that can be adjusted by the respective type of resin can be modified within a wide range.

It is known in the art to apply resins to the fiber suspension prior to sheet formation. For example, in U.S. Pat. No. 4,810,430 and U.S. Pat. No. 6,077,391, resins are added to the fiber suspension to adjust filtering properties. However, no thermosetting resins are applied to papers known in the art. Resins in papers known in the art are not plastified and cured by applying high temperature and pressure after the paper has been dried.

Suitable thermosetting resins that can be processed in an aqueous dispersion preferably have one or more of the following properties: Alkalinity should be between 2% and 5%, advantageously between 2.5% and 4.5%. It is therefore lower than the alkalinity of phenolic resins, used for impregnation according to the art (alkalinity 6% to 8%). Viscosity should preferably be between 250 mPas and 1000 mPas. Advantageously, thermosetting resins that are used in the manufacturing of resinous papers according to the invention can have a viscosity from 600 mPas to 900 mPas. Therefore, thermosetting resins, e.g. phenolic resins, can be used in a significantly wider range of viscosity than before (hitherto: viscosity has varied from 250 mPas to 500 mPas). Advantageously, the free formaldehyde content of the thermosetting resin is up to 1.7%, preferably up to 1.5%, particularly preferably up to 1.0%, especially up to 0.5%, particularly advantageously up to 0.2%. Therefore, thermosetting resins are used with a free formaldehyde content that is far lower than that of thermosetting resins used to date, which usually have a free formaldehyde content of approx. 2%. Synthetic resins that are preferred according to the invention have an acetone extraction between 10% and 20%, preferably, the acetone extraction is between 10% and 15%. Resin flow is between 4% and 12%, preferably between 5% and 8%.

The resin is present in the aqueous dispersion in a very finely dispersed form, so that it is uniformly mixed with the fibers. The fibers and the resin are first present together without there being a bond between the two components. The suspension is then adjusted in its acidity to a pH value below 5, preferably to a pH value between 3 and 4, for example by the addition of sulfuric acid. The pH value is adjusted by adding acids, e.g. by adding sulfuric acid, hydrochloric acid or organic acids. The lowering of the pH value leads to a precipitation of the phenolic resins. The precipitation of resins does not yet lead to a bond with the fibers.

According to the present invention, an agent for fixing the thermosetting resins is mixed into the suspension. By this measure the charging surfaces of the resin and the fibers are configured in such a way that the resin is deposited on the surfaces of the fibers. The agent for fixing is known in the art. It is preferably added to the suspension before the pH value is lowered. The agent(s) for fixing is/are selected primarily with a view to fixing the thermosetting resins as completely as possible on the fiber surfaces so that as little resin as possible, in particular as little phenolic components as possible, remain in the waste water (current limit: 100 mg/l). When the agent for fixing has taken effect, the fibers of the suspension are at least partially coated or covered with synthetic resin. It is not necessary that the fibers are completely covered with synthetic resin. What is essential is that the amount of synthetic resin to be applied is precipitated on the surface of the fibers as completely as possible, so that the fibers and the synthetic resin are attached to each other prior to sheet formation.

The thus processed fiber material can then be transferred via the conventional headbox to a paper machine, where the coated or resinous fibers are formed into sheets or paper by dewatering (wet laying). Hereby, a paper or a preform blank is formed in which at the crossing points between the fibers in at least some contact points, often in the majority of the contact points, fibers do not get into direct contact with each other. Rather, there is synthetic resin in between the fibers, since at least one of the intersecting fibers is coated with synthetic resin. The paper is then adjusted to a residual volatile content of below 10%, but above 5%, in the drier section of the paper machine. To obtain optimum reactivity of the resinous paper, mostly a paper volatile content of 6% to 8% is adjusted. The reactivity of the paper is a measure for the ability of the thermosetting resin to cure completely under pressure and heat.

It must be seen as a particular advantage of the present invention that resinous papers having a substantially wider range of weight can be manufactured by the method of the present invention, in particular resinous papers having an overall weight from 140 g/m² up to 1000 g/m². Where it has hitherto been necessary to press a large number of thin superimposed core papers onto each other, the number of core papers can be reduced to a few, in particularly advantageous cases to a single, but heavier core paper, which is a simplification in particular with respect to the manipulation of the individual layers of material and paper. Both light and heavy inventive core papers can be used to produce high-quality coatings using common process conditions:

For example in the coating of window sills or work surfaces of wooden materials, which require particular surface protection, the number of core papers can be lowered from 50 to between 10 and 15. This reduces mechanical handling for depositing the core papers on the wood material panel and is reduced to about a quarter. At the same time it has been found that a typical problem is solved which often occurs when a great number of core papers are to be pressed. Prior art papers, resinous on their surfaces, tend to form voids during pressing; individual papers do not stick to each other and are not bonded to a uniform layer during pressing. Void formation leads to waste. During pressing of the core papers where resin was added to the suspension, according to the present invention, such phenomena do not occur which is presumably due to the core papers having a more homogeneous bond between the fibers and the synthetic resin.

A further advantage is that the fibers and the synthetic resin are now more uniformly distributed; the core paper according to the present invention is more homogeneous overall. The synthetic resin is now firmly bonded to the fibers, which avoids “blocking” (accumulation of resin).

A yet further advantage is that by using the resinous paper or the preform blank of the present invention the cycle time may be reduced by 15-25%, preferably up to 30%.

A still further advantage is that by using the resinous paper or the preform blank of the present invention there is no need for a backing paper to avoid sticking of the product, for instance in rolling, because resin will not flow out of the paper or preform blank as it does easily with impregnated paper

According to an advantageous further development, the content of the thermosetting resin in the paper is 30 weigth % to 50 weight % of the solids content of the paper. Since the fiber content can be used in a substantially wider range of weights, as a result, despite the otherwise unchanged percentage by weight, the resin amount used can be varied more widely than with prior art core papers.

In a preferred embodiment of the invention, the synthetic resin used can be dispersed in water, and is preferably water-soluble. The synthetic resin is typically provided with a solids content of 50% +/−2% in an aqueous solution. It can be further diluted based on the delivered form, up to a ratio of resin to water of 1:20. The inventive method is flexible regarding the solid content of the synthetic resin, since it is used for wet laying highly diluted anyhow. This diluted resin dispersion or resin solution is easily dosable and can be quickly and uniformly mixed with the fiber mass suspension, which is particularly advantageous in a continuous manufacturing process.

According to the invention, agents are required for fixing the resin on the surface of the fibers. These agents act by means of charging the surfaces involved and therefore cause different charges on the surfaces, so that the resin is deposited and fixed on the oppositely charged fiber surface. In paper manufacture mostly cationically acting agents are used for fixing, but anionic, non-ionic and bi-ionic fixing agents are also well known and available. Typical examples of cationic fixing agents are high-molecular polyethylene imines or condensated organic amides with formaldehyde. Such agents for fixing can be used alone or in a mixture together with each other.

It is to be understood as an advantage of the present invention that fire-protection additives can be added to the fiber suspension prior to sheet formation. These additives can also be very uniformly distributed when they are added prior to sheet formation. This is important in particular with high sheet weights of the resinous paper, because the fire-retardant characteristics of the paper are particularly good if the fire-protection additives are uniformly distributed, so that there are no weak points. The usual fillers, such as chalk, kaolin and the like, already have a fire-retardant effect, because they are non-combustible materials. While these fillers cannot be introduced into prior art core papers in a similarly efficient manner, with the method according to the present invention, it is possible for the first time to manufacture resinous papers compliant with the requirements of the B1 fire rating. Typical examples of flame-protection or fire-protection additives are aluminum tri-hydrate, chloroprene latex or mixtures of carbamides, organic sulphonic acids and inorganic salts.

If desired or necessary, other additives, that are e.g. necessary for sheet formation or other purposes, can be added to the aqueous suspension.

Another object of the invention is a resinous paper. The inventive paper is characterized in that the fibers and the thermosetting resin are related to each other in a different way than in known papers, especially core papers. The fibers of the inventive paper are at least partially coated with a thermosetting resin, e.g. phenolic resin, melamine resin or urea resin. Coating with the resin is carried out prior to sheet formation. The fibers thus coated with resin attach to each other during sheet formation in contact points in such a way that in at least some of the contact points resin-coated sections of the fiber surfaces rest against each other. Preferably, resin-coated sections of the fiber surfaces rest against each other in the majority of the contact points.

It must be highlighted as a special advantage of the invention that both light and heavy papers can be produced from the fibers at least partially coated with thermosetting resin. The inventive papers can be manufactured in a wider weight range than papers known in the art. The amount of thermosetting resin applied to the fibers can be varied within a wide range, as has been depicted above by the example of the inventive process. It is possible to produce papers with a grammage up to 300 g/m², as is possible with papers according to the art. However, it is also possible to produce heavier papers, in particular with a grammage of more than 400 g/m², preferably more than 500 g/m², particularly preferably of more than 700 g/m², advantageously of more than 900 g/m², especially preferably of up to 1000 g/m². The grammage of the inventive paper can basically be adjusted within a wide range; it is mainly limited by the amount of thermosetting resin applicable to the fibers and the workability during sheet formation as well as during pressing of the resinous paper.

The fibers of the inventive paper can be natural fibers as well as synthetic fibers or mixtures of these. Fibers can be used for the inventive paper if they can be processed by wet laying and if thermosetting resin can be precipitated onto the surface of the fibers.

The amount of thermosetting resin that can be applied to the resinous paper is up to 50 weight % based on the solid content of the paper. It can also be adjusted to a lower value, e.g. 30 weight % based on the solid content of the paper. The upper limit of the amount of thermosetting resin is determined by the adsorption capacity of the available fiber surface. The lower limit is determined mainly by requirements on the use of the resinous paper. For technical reasons, small amounts of thermosetting resin can be applied as well. However, in order to produce a resilient, usable surface on the material coated with the resinous paper, a thermosetting resin content of 30 weight % or more is usually necessary.

After sheet formation, the thermosetting resin has dried to the extent that it is blockfree. It is, however, reactive at elevated temperatures (e. g. 160° C.) and under elevated pressure (e. g. 80 bar) and is only cured completely under these conditions.

If a phenolic resin or a mixture of synthetic resins containing a phenolic resin is used as the synthetic resin, it was found detrimental in known core papers that they exhibit a strong inherent smell of phenol that is disturbing during manufacture, in particular during pressing. Free phenol content of most core papers is measured at more than 3 mg/l, free formaldehyde at more than 4 mg/l (comparative measurement). Surprisingly, this drawback is not encountered with the paper produced with phenolic resin according to the invention. It can be manufactured almost free of smell. When the inventive paper is pressed onto a high-density fiber board to produce a laminate, free phenol is measured at 0.37 mg/l, free formaldehyde at 2 mg/l. This is an extraordinary improvement with respect to the state of the art.

As an independent part of the present invention, the use of the resinous paper as a resinous preform blank has also been claimed which preform blank is used for the production of formed objects. The inventive preform blank can have the same technical properties as the paper described above. As described above, a particularly thick, sheet-like preform blank having a weight of up to 1000 g/m² can be manufactured according to the present invention, which preform blank is composed of fibers and thermosetting synthetic resin, dried, but not yet hardened. This preform blank can be inserted in molds, for example, and shaped and hardened under the effect of pressure and heat to produce two- or three-dimensional molded parts, which can be used, for example, in the automotive industry, in the construction of housings or items of daily use and for numerous other applications. The conditions for curing are the same as during pressing of the papers according to the present invention on a substrate. Unlike comparable molded parts, which are manufactured in an injection molding method according to the prior art, the preform blank according to the present invention can be more easily handled, for example, it can be cut to size or provided with holes prior to insertion into the mold. In particular, injection of the mass to be molded into the injection mould, which can be bothersome, can be eliminated.

It follows from the preceding that the inventive preform blanks that consist of fibers whose surface is at least partially provided with a thermosetting resin can be used for a wide range of applications. They can be used for indoor and outdoor construction, as furniture or parts thereof, as housings, as casings, for automotive construction, for mold making and similar purposes, while the grammage and the simple and variable workability by pressing makes it applicable not only for two-dimensional, but also particularly for three-dimensional objects.

Details of the invention will be explained below with reference to two embodiments.

To produce resinous papers with a grammage of 140 g to 500 g, first, a fiber suspension is created. For this purpose 700 kg unbleached softwood kraft pulp is mixed into 16 m³ of water. If necessary, the fiber material in the suspension is ground to a SR degree between 30° and 35°. 450 kg aluminium hydroxide is mixed into 40 m³ water. Fiber suspension and aluminium-trihydrate solution are blended and 150 kg of a polychlorbutadiene polymer is added as retention means. Finally, 1050 kg of a phenol-resol synthetic resin in an aqueous-alkaline solution with a solid content of 48% is added. The viscosity of the synthetic resin is between 600 mPas and 900 mPas (viscosity at 20° C.; DIN 53015). The alkalinity of the resin is 4.0% (DIN 169 16-02-G) and components that are precipitatable by sulfuric acid with a pH value of 3,5 amount to at least 30%. The synthetic resin may be diluted up to a resin-to-water ratio of 1: 20 (DIN 169 16-02) i.e. the resin concentration is at least about 5%. The phenol-resol resin is acid active and heat reactive. It has a low monomer content. After addition of the phenol-resol synthetic resin, the pH value of the fiber suspension is adjusted to 3.5 using 20% sulfuric acid. After addition of 3 kg of a commercial defoamer (Afranil® MG ex BASF), the fiber concentration is adjusted to approx. 3%.

Thus prepared, the fiber suspension is dewatered on a paper machine wire with a web-width of approx. 2.30 m at a wire speed of 35 m/min to a paper with a grammage of 500 g/m² OD. A carbamide-containing combination of organic sulfonic acid with inorganic salts as flame retardant is applied in an amount of 50 g/m² to the dewatered, but not completely dried paper having a volatile content of approx. 40%. The paper thus treated is dried at a temperature of 120° C. to a residual volatile content of 6% to 9%.

The thus produced paper can, for example, be put on a wooden material board and pressed 20 to 25 minutes at 130° C. and 60 bar in a press. If pressing times are to be shortened, pressing can be carried out at e.g. 160° C. to 180° C. and 80 bar.

To produce a further resinous paper of 180 g/m², 65 g OD hemp fibers having a SR degree of 35° SR and 30 g OD of a phenol-resol synthetic resin are provided according to the above described embodiment; into this aqueous suspension having a fiber concentration of 5% are added 2,5 g of a commercial defoamer (as in example 1 Afranil® MG of BASF) and 1 g of a cationically acting retention means, e.g. Catiofast® of BASF. The suspension is adjusted to a pH value of 3.5 using sulfuric acid. From this fiber suspension, a sheet having a grammage of 180 g/m² is made.

Round sheets are punched out of the 180 g/m² square sheets. The round sheets are piled upon each other and are subsequently cured under pressure in a convection oven at 160° C. for approx. 2 hours. Thus, a roll having a hardness of approx. 90 ShoreD is formed. The surface of the roll is smoothened by grinding or turning. These kinds of rolls that are made as formed objects from resinous papers are for example used for finishing in paper or textile industry.

All percentages in this document are to be understood as weight percentages, unless otherwise specified. The specification “OD” in this document relates to “oven-dry” material, dried at 105° C. until constant weight is achieved. Grammage was determined according to Zellcheming Vorschrift V/11/57. The degree of grinding was determined according to Zellcheming-Merkblatt V/3/62. 

1. A method of manufacturing resinous paper, comprising the steps of: preparing an aqueous fiber suspension having a consistency between 0,5-8%, preparing an aqueous dispersion of a thermosetting resin mixing the aqueous dispersion of the thermosetting resin into the aqueous fiber suspension, preparing an aqueous dispersion of an agent for fixing the thermosetting resin on the fibers of the fiber suspension, mixing the aqueous dispersion of the fixing agent into the aqueous fiber suspension, adjusting a pH value of the aqueous fiber suspension to below 5, transferring the aqueous fiber suspension to a paper machine, dewatering the fiber suspension for the formation of paper, drying the paper to a volatile content of 5% to 10%.
 2. The method according to claim 1, characterized in that the content of the thermosetting resin in the paper is 30 weight % to 50 weight % solids content in the paper.
 3. The method according to at least one of the preceding claims, characterized in that the thermosetting resin is water-dispersible or water-soluble.
 4. The method according to at least one of the preceding claims, characterized in using phenolic resins, melamine resins or urea resins, or mixtures of these resins as the thermosetting resin.
 5. The method according to claim 1, characterized in using cationically, anionically, non-ionically and/or bi-ionically acting substances as the fixing agent for fixing the resin on the fibers.
 6. The method according to at least one of the preceding claims, characterized in adding fire-protection additives to the suspension.
 7. The method according to claim 6, characterized in adding one or more of aluminum-trihydrate, chloroprene-latex and/or mixtures of carbamides with organic sulphonic acids and inorganic salts as fire-protection additives to the suspension.
 8. The method according to at least one of the preceding claims, characterized in manufacturing a paper having a grammage from 140 g/m² to 1000 g/m².
 9. The method according to at least one of the preceding claims, characterized in drying the paper under heat.
 10. A resinous paper containing fibers coated with a thermosetting resin and manufactured by the method according to at least one of claims 1 to
 9. 11. The resinous paper according to claim 10, characterized in that fire-protection additives are added to it and it is classified in fire-protection class B1 (B1 fire rate).
 12. The resinous paper according to claim 10, characterized in having fibers arranged such that they intersect at contact points and that at least at some of the contact points the fibers are coated with thermosetting resin.
 13. The resinous paper according to claim 10, characterized in that it has a weight of up to 300 g/m².
 14. The resinous paper according to claim 10, characterized in that it has a weight of more than 400 g/m², preferably more than 500 g/m², particularly preferably more than 700 g/m², advantageously more than 900 g/m², especially advantageously up to 1000 g/m².
 15. The resinous paper according to at least one of claims 10 to 14, characterized in that it has a thermosetting resin content of 30 weight % to 50 weight % based on the solid content of the paper.
 16. The resinous paper according to at least one of claims 10 to 15, characterized in that it is a singe-layer or a multi-layer paper.
 17. The resinous paper according to claim 16, characterized in that it is a multi-layer paper with a grammage of more than 400 g/m² and with a thermosetting resin content of 30 weight % to 50 weight % based on the solid content of the paper.
 18. The resinous paper according to at least one of claims 13 to 18, characterized in that it is a coated or an uncoated paper.
 19. Use of the resinous paper according to at least one of claims 10 to 18 as a resinous preform blank comprising fibers coated with a thermosetting resin. 